Glass laminated bodies comprising a tensilely stressed core and a compressively stressed surface layer fused thereto

ABSTRACT

This invention relates to sound, high strength, laminated articles of glass, glass-ceramic, glass and glass-ceramic materials. Such articles are made by means of a continuous hotforming process wherein glasses are melted for the individual layers and these layers are then simultaneously fused together and shaped into a laminated structure of a desired configuration. Where a glass-ceramic article is desired, the laminated glass structure is subsequently heat treated in a particular manner to cause the glass to crystallize in situ.

United States Patent Giffen et al.

1 June 27, 1972 GLASS LAMINATED BODIES COMPRISING A TENSILELY STRESSEDCORE AND A COMPRESSIVELY STRESSED SURFACE LAYER FUSED THERETO Inventors:James W. Glifen; David A. Duke, both of Coming; William H. Dumbaugh,Jr., Painted Post; James E. Flannery, Corning; John F. MacDowell,Painted Post; John E. Meglea, Corning, all of N.Y.

Assignee: Corning Glass Works, Corning, N.Y.

Filed: Oct. 7, 1970 Appl. No.: 70,763

Related U.S. Application Data Continuation-impart of Ser. No. 735,074,June 6, I968, abandoned.

U.S. Cl ..l6l/164, 65/41, 65/114, 65/121, 65/145, 106/39 R, 106/48,106/54,

Int. Cl. ..B32b 7/02, B32b 17/06 Field ofSearch.. ....161/1,43,193,166,164,l65; 65/41,114,145,121;!06/54, 39 DV, 48, 52, 53,

39 R; 117/123 A, 124 A, 125

I References Cited Primary Examiner-Robert F. Burnett AssistantExaminerJoseph C. Gil Anomey-Clarence R. Patty, Jr. and Clinton 8..lanes, Jr.

[57] ABSTRACT This invention relates to sound, high strength, laminatedarticles of glass, glass-ceramic, glass and glass-ceramic materials.Such articles are made by means of a continuous hot-forming processwherein glasses are melted for the individual layers and these layersare then simultaneously fused together and shaped into a laminatedstructure of a desired configuration. Where a glass-ceramic article isdesired, the laminated glass structure is subsequently heat treated in aparticular manner to cause the glass to crystallize in situ.

15 Claims, 4 Drawing Figures PATENTEDJUHZ? m COMPRESSION I TENSIONCOMPRESSION TE NSION TENSION ITENSION COMPRESSION COMPRESSION David A.Duke Jade mmnwzfif WEDM E UFOO. N b e W M/ D E n e A mmm wdw BACKGROUNDOF THE INVENTION In the past, glass has been thought of as a weak andbrittle material. Although glass is a brittle material, it cannot trulybe classified as a weak one. Glass normally fails in tension as a resultof surface defects. Therefore, many attempts have been made tostrengthen glass by providing it with a surface layer which is incompression.

A mid seventeenth century curiosity known as the Prince Rupert Drop wasamongst the first reported strengthened glasses. The basic mechanism,although not known at the time, has since been defined and is now knownas tempering. Tempering comprises rapidly cooling a glass object so asto establish a temperature gradient therein under conditions where theglass is sufficiently low in viscosity to yield and release temporarystresses. As the object is cooled to room temperature, the temperaturegradient originally established disappears, and a state of stress iscreated with the central section of the object in tension and the outersurface section in compression. This surface compression increases thestrength of the body. The degree of strengthening will depend upon thetemperature from which the body was cooled and the rate of cooling. I

There are several chemical techniques by which glass articles may bestrengthened, all of which are relatively new. One such techniquecomprises contacting the surface of a sodium or potassium silicate glassarticle, at a temperature above the strain point of the glass, with anexternal source of lithium ions. This contact causes the lithium ions toreplace the sodium or potassium ions in the surface of the glassyielding a surface laying have a lower coefficient of thermal expansionthan the parent glass. .Thus, when the body is cooled below the strainpoint of the glass, the higher expansion interior contracts more thanthe lower expansion exterior leaving the low expansion surface layer ina state of residual compression.

A second chemical strengthening technique has been developed whereinlarge potassium ions from a salt bath are exchanged for smaller sodiumions in the glass at temperatures below that at which the glass can flowand relieve the stresses. Therefore, the introduction of the potassiumion into the positions previously occupied by the sodium ion results ina crowding of the surface. This crowding creates a rather high residualcompressive stress in the surface and a counterbalancing tensile stressin the interior.

Strengthening by the use of an overlay is also known to the I art. Anexcellent example of this is found in US. Pat. No.

2,157,100 wherein the patentee teaches a method of strengthening aceramic insulator by applying a glaze having a coefficient of thermalexpansion approximately percent less than that of the ceramic body. Uponcooling, the glaze is left in a state of residual compression therebyeffectively increasing the strength of the whole body. This technique iswell-known and documented throughout the china-body industry, and is thetypical method for strengthening dinnerware. Still higher strengthbodies have been developed by glazing glass-ceramic articles. Specialglazes have been applied to glass-ceramics so that upon maturing, acrystalline interlayer is formed between the glass-ceramic and glaze.This interlayer permits greater differences in the coefiicients ofthermal expansion and thus higher strength bodies. These glazes andtheir application to glass-ceramic articles are described in US. Pat.No. 3,384,508.

In 1891, Otto Schott made boiler gauge glasses by overlaying a highexpansion glass with a low expansion glass. He did this by inserting aniron rod into molten high expansion glass, gathering a gob of this glasson the rod, cooling it slightly, and then inserting it into a second potof molten low expansion glass. He then drew the composite glass into arod. Upon cooling, the low expansion exterior glass was left in a stateof residual compression, thereby strengthening the composite.

US. Pat. No. 1,960,121 teaches a process for forming a strengthenedcomposite glass article wherein the index of refraction of the articleis the same throughout. The disclosure in that patent indicates that amethod of strengthening glass by utilizing two or more layers of glasseshaving difi'erent coefficients of thermal expansion is well known in theart. The disclosure also indicates that the relationship between thethickness of the various layers and the coefficients of expansion of thevarious glass layers is also well known and that the lower expansionlayer is always the thinnest of the layers. Furthennore, the patentteaches that the layers should be united while they are still soft. ABritish Patent, No. 405,918, teaches that it is known to join togethertwo or more laminae of fluid glass having difi'erent coefiicients ofexpansion. However, the disclosure indicates that there are problems incutting and forming of these bodies. Therefore, it suggests that two ormore sheets having the required coefficients of expansion be cut in thecold state and then heated to a temperature at which their viscositieswill be between 10 and 10 poises. Thereafter, the separate sheets can bepressed or rolled together so as to form a laminated sheet. By thisprocess, a strengthened laminated sheet wherein no problems ofcontrolling the size and shape thereof can be produced.

From the above references, it is evident that the general concept ofproducing a strong laminated composite body, wherein the layers are ofdifferent coefficients of thermal expansion, is well known. However, todate there has been no true commercial production of hot laminated glassbodies. Furthermore, there appears to be no consistent definition ofparameters necessary to produce such bodies. For example, although it isstated that there should be a difference in coeflicient of expansion,there are no teachings as to what these differences should be.Similarly, it is stated that there should be differences in thethicknesses of the layers, but again there is no teaching of thethicknesses which are permissible and their relation to the coefficientof expansion. Neither is there any teaching as to the interrelation, ifany, between the differences in the coefi'rcients of expansion and thethicknesses of the layers. It is also stated that the glasses should besoft but there is no disclosure of the viscosities thereof or thetemperatures at which such bodies are formed. Neither is there astatement as to the relationship, if any, between the viscosities of thedifferent layers. Thus, there is no teaching in the art as to how toprepare a hot-laminated strengthened glass body. Neither is there ateaching of how to prepare the laminate in a continuous, rather than abatch, process.

The prior art does not teach the stress distributions and magnitudesthereof which will produce a desirable laminated body. The distributionand magnitude of the stresses will determine the strength of the bodyand the violence of breakage of such a body. The only teaching availableis US. Pat. No. 2,177,336 which relates to tempered glass bodies.However, the teaching therein is not applicable with respect to what isnecessary in a laminated system, for a number of reasons. Tempered glassmust be relatively thick in order to achieve strengthening, which is notthe case with laminated glass. The stress distribution in tempered wareis significantly different than in laminated ware. These differences arereadily seen when FIGS. 1 and 2 and FIGS. 3 and 4 are compared.Furthermore, much higher ratios of maximum compression to maximumtension can be obtained in laminated ware than in tempered ware. Thus,the violence of breakage of tempered ware will generally be greater thanthat with laminated ware, since for the same maximum compressive surfacestress the tempered ware has a much higher interior tensile stress.Finally, although various physical properties are discussed in the art,there is no indication as to the glass compositions, or familiesthereof, which would produce acceptable laminated glass bodies forcommercial use were not solved by the prior art.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a representation of thecross-sectional stress distribution in a tempered body.

FIG. 2 is a representation of the cross-sectional stress distribution ofa laminated body having the same maximum internal tensile stress as thebody in FIG. 1.

FIG. 3 is a representation of the cross-sectional stress distribution ina tempered body.

FIG. 4 is a representation of the cross-sectional stress distribution ofa laminated body having the same maximum compressive stress as the bodyof FIG. 3.

SUMMARY OF THE INVENTION We have invented a strengthened laminate systemfor the production of glass, glass-ceramic, or glass and glass-ceramicbodies which are relatively inexpensive in composition, readilyproducible commercially, and do not exhibit high violence of breakage.The basic unit of this invention is comprised of a tensilely stressedcore portion having adhered thereto and being substantially enveloped bya compressively stressed layer. The laminate of our invention can assumemany forms such as a sheet, a rod, a sphere, etc. In the case of a sheetor shapes made therefrom, the laminate can be considered to be made upof three plies, a core ply being tensilely stressed and having twocompressively stressed plies adhered thereto. These compressivelystressed plies correspond to the layer described above. In all shapes,it is preferred that the compressively stressed layer should completelyenvelop the composite article; however, exposure of a small portion ofthe inner structure will not deleteriously affect the laminate of ourinvention.

We have found that the laminate of this invention must be formed atelevated temperatures so as to obtain intimate bonding, or fusion, ofthe core and adhered layer. The formation at elevated temperatures isadvantageous in that the glass surfaces when fused are virgin or defectfree. These glasses have not been handled and are so fluid that anysurface defects will heal. Thus, the surfaces have not been mechanicallydegraded. At the lamination temperature, the viscosity of the core andadhered layer must bear a particular relation to each other. Thus, atthe lamination temperature the core must be between about 1 to 6 timesas viscous as the adhered layer. Furthermore, the liquidus temperatureof the glasses must be low enough to avoid devitrification duringlaminating. In order that the laminate will be strengthened, the adheredlayer of the laminate must have a coefficient of thermal expansion atleast 15 X 10'/ C. less than that of the core at the setting point ofthe softest glass in the laminate. However, if the laminate iscompletely glass-ceramic the coefficient of thermal expansion of theadhered layer may be at least 5 X l0"/ C. less than that of the core atthe setting point of the softest glass-ceramic in the laminate.Moreover, to insure proper strengthening and breakage characteristics,the ratio of core thickness to total adhered layer thickness (i.e., thetotal of the core and adhered layer thicknesses when viewed in crosssection) should be between about 1021 and 30:1.

DESCRIPTION OF THE PREFERRED EMBODIMENT We have found that three-plylaminated articles are particularly desirable and when made inaccordance with this invention derive many of their beneficialproperties from a unique stress distribution which can be obtained onlyin a laminated body. The cross-sectional stress distribution can becharacterized as rectilinear. That is, the compressively stressed outerply of the body always experiences about maximum compression whiletensilely-stressed core ply always experiences about maximum tension.This is observed in FIGS. 2 and 4. Thus, very high compressive stressescan be introduced into the adhered plies while the magnitude of thecounterbalancing internal tension is relatively low. For example, theratio of maximum compression to maximum tension can be on the order of20: 1; whereas in a thermally tempered body that ratio may be about 2:1.Thus, for the same maximum tension much greater maximum surfacecompression can be obtained in thev laminated system than in a temperedglass as is seen when FIGS. 3 and 4 are compared. Violence of breakageis related to total tensile strain energy in the body, which is in turnre-, lated to some extent, to the maximum tensile stress and relativethicknesses of the core and adhered plies. Therefore, in bodies of thesame thickness having the same compressive stress the violence ofbreakage is lower in the laminated body than in the tempered body, sincethe maximum tensile stress is lower in the laminated body than in thetempered body. Furthermore, the maximum stresses developed inthelaminate are related to the ratio of the thicknesses of the core toadhered plies and not to the absolute thickness. This is opposed to atempered body wherein the stresses are related to the actual thicknessin addition to the thickness ratio.

We have found that, for the preferred bodies of our invention, theflexural strength of the laminate as measured in terms of modulus ofrupture (MOR) should be between about 25,000 and 50,000 psi. Ifthe MORis below about 25,000, the bodies will not be strong enough to resistsevere mechanical impact. However, if the MOR is above about 50,000 psi,failure of the preferred bodies by breakage can be quite violent.Strengths of this level are attained by controlling the differencebetween the coefficient of thermal expansion of the core ply and adheredplies, otherwise known as the expansion mismatch, and by controlling thethickness of the respective plies.

In a laminated system, one factor in determining the stress in thelaminate is the amount of strain in the object. The strain is determinedat the setting point of the softest material in the laminate. Thesetting point of a glass is defined as a temperature 5 C. above thestrain point. The stress in a body can then be calculated using, amongother things, the strain. Rather than actually measuring strain, a goodapproximation thereof is the difference in the coefficient of expansionof the glasses as measured from 25 C. to 300 C., multiplied by thetemperature differential from the setting point tothe use temperaturevAnother way of viewing this is that there must be at least some minimumdifference in expansion, between the glasses, at the setting point ofthe system. In laminates having at least one glass ply, the difl'erencein expansion, or expansion mismatch, which is necessary in order toattain the above mentioned strength levels is at least about 15 X l0"/C. However, where the laminate is all glass-ceramic the difference mustbe at least 5 X l0 C. Furthermore, these differences in expansion areinterrelated with the thickness of the various plies. In normalcommercial practice, the coefficient of thermal expansion for the coreply should be between about 60 and 110 X l0"/ C.; the coefficient ofthermal expansion of the adhered plies should be between about 30 and Xl0"/ C. In our preferred embodiment, which has at least one glass ply,the coefficient of expansion of the adhered plies is approximately 46 Xl0"'/ C. and the coefficient of thermal expansion of the core ply isapproximately 67 X l0/ C. This results in an expansion mismatch of about21 X 10 C. which is in excess of the minimum mismatch of 15 X l0"/ C.

A very important factor in determining the strength of the body is thethickness ratio of the core ply to adhered plies. For the preferredembodiment, with the aforementioned coefficients of expansion, andexpansion mismatches, we have found that the thickness ratio should bebetween 10:1 and 30:1, preferably 15:1. Ifthe ratio is less than 10:1,for example 5:1, there would be a concomitant high violence of breakage.This is related to the high tensile stress in the core and the resultantsensitivity to material defects, such as stones. However, if the ratiowere greater than 30: l for example 40: 1 the adhered plies in thepreferred body would be relatively thin and, as a result, normalhandling could introduce surface defects which could penetratetherethrough. Hence, in the preferred embodiment, a thickness ratio ofgreater than 30:] could result in mechanically non-durable bodies.However, if the surface were protected so that a mechanically durablesurface were not essential, the thickness ratio could be in excess of30: l and still maintain the desired high compressive stress.

In the preferred embodiment, the actual total thickness of the adheredplies is about 0.006 inch, with about 0.003 inch on either side of thecore and the core thickness is about .070 inch minimum. This results ina thickness ratio for the body of the preferred embodiment of 70:6, orabout 12:1. Thus, the body formed with the aforementioned preferredthicknesses, thickness ratios, expansions, and expansion mismatches willresult in a laminated body wherein the compressive stress in the adheredplies'is approximately 30,000 psi.

The glass laminate of this invention may be used for tableware;therefore, it is desirable that, in case of failure, the laminated bodydoes not fail violently. In other words, the violence of breakage shouldbe low. The violence of breakage is related to the amount of tensilestrain energy within the laminate. The amount of tensile strain energyis related to the expansion mismatchand the thickness of the plies. Ifthe tensile strain energy is high, the violence of breakage will also behigh. The normal mode of a violent break is dicing, or fracture, of thebody into many small pieces. It is quite obvious that it is undesirableto have these small pieces flying about. Therefore, the tensile strainenergy ought to be maintained at a relatively low level but sufficientto allow strengthening of the body. Delayed breakage is another problemwhich is related to strain energy; that is, a flaw may unintentionallybe introduced into the surface and failure from that flaw will notinitiate until some finite time after the introduction thereof. Thisfailure could occur even as the user is opening the cupboard to remove apiece of previously stored tableware. To avoid this delayed breakage,the level of tensile strain energy is maintained at a relatively lowlevel. Thus, it is not desirable to strengthen these particular bodiesbeyond about 50,000 psi since above that stress level the tensile strainenergy is considered to be sufficient to cause violent breakage ordelayed breakage. On the other hand, it is sometimes permissible toproduce articles which have a high violence of breakage, as for example,in building tiles, roofing tiles, or wall tiles. This high violence ofbreakage is controlled by providing the body with a high level ofinternal tensile strain energy. This level of tensile strain energy isprovided through the selection of appropriate ply thicknesses, thicknessratios, expansions, and expansion mismatches. Thus, with this system oflamination, bodies having a predetermined level of violence of breakagecan be produced.

One of the primary advantages associated with this laminating system isthat the weight of the body may be low since the strength level is highwithout a large cross-section. Thus, by the appropriate choice ofcoefficients of thermal expansion, expansion mismatch, thickness ratios,and actual thicknesses, a sound, strong, light-weight laminated body maybe produced.

The three-ply bodies of the preferred embodiment are fabricated byforming a laminated sheet, shaping the laminated sheet, and then cuttingout the desired shape. The core ply may be exposed along the cut edgeduring the cutting step; however, this exposure is not preferred since astronger body can be produced when the core ply is completely enveloped.By appropriately designed cutters, the amount of exposed core ply can beminimized; furthermore, other secondary operations can completelyenvelop the core ply. In order to form the laminated sheet, it isnecessary that at the moment of lamination, the viscosity of the coreply and the adhered plies be in a ratio of from about 1:1 to 6:1. Wehave found that the sheet may readily be formed when the viscosity ratiois between about 25:1 and 3.5: 1. Normally, during the laminatingoperation the core and adhered plies are maintained at the sametemperature while maintaining the desired viscosity ratio. However, thecore and adhered plies can be at different temperatures when laminatedso long as the viscosities are within the required ratio. On an absolutescale for the preferred embodiment, the viscosity of the core ply isapproximately 4,000 poises and the viscosity of the adhered ply isapproximately 1,500 poises, at the laminating temperature. Thisviscosity ratio is about 3:1. The selection of the absolute viscositiesis' related to the particular laminate forming technique used. Forexample, if an updraw or downdraw process were used or shapes other thana sheet were made, a different set of viscosities would be selected;however, the ratio of viscosities would remain within the aforementioned1:1 to 6:1 range. Hence, in tube or cane drawing processes, viscositiesbetween about 50,000-200,000 poises may be required; whereas, in anupdraw process for glass sheet, a viscosity of 100,000-250,000 poisesmay be required.

We have found that the most convenient laminating temperatures, for thelaminate of the preferred embodiment, are in the range of between aboutl,225 C. and l,325 C. The preferred laminating temperature is about1,275 C. The liquidus temperature of the glasses used for the core andadhered plies should be low enough so as to avoid devitrification duringlaminating. in view of the range of laminating temperatures, theliquidus temperature can be as high as l,300 C.;

however, we prefer to use glasses having a liquidus temperature belowabout 1,200 C.

It may be desirable to heat treat the laminated body formed by theaforementioned process. Normally, the heat treating temperatures aremaintained below 850 C., since above that temperature the bodies tend todistort. Above 850 C., it is necessary to heat treat the bodies informers so as to maintain their shapes. In order to help preventdistortion of the bodies during heat treatment below 850 C., it isdesirable that at some temperature below the laminating temperature, butabove the heat treating temperature, there should be a reversal of theviscosity relationship at lamination. Thus, at temperatures less thanthe maximum heat treating temperature, or 850 C., the viscosity of theadhered plies will be greater than that of the core ply. This higherviscosity tends to prevent distortion of the body during heat treatment.Thus, at the heat-treating temperatures the glass of the adhered pliesis more viscous than the core glass. In addition to helping preventdistortion, this higher viscosity tends to prevent deformation of theadhered ply which is in contact with the lehrbelt, or metal conveyorbelt, which carries the body through the heat treating furnace.Similarly, the adhered plies should have a high annealing point, atleast 600 C., so that during heat treatment deformation of adhered pliesis further prevented. in the preferred embodiment, the annealing pointshould not be lower than 700 C. However, if there is no subsequent heattreatment, the annealing point of the adhered plies can be below 600 C.

The composition range for glasses for the adhered layer, or plies, whichmeet the aforementioned requirements, in weight percent on the oxidebasis as calculated from the batch, is disclosed below. SiO should bepresent between about 50-65 percent. Below 50 percent, the annealingpoint is too low and the expansion tends to be too high. Above 65percent, the viscosity in the forming range is too high. A1 0 should bepresent from about 10 to 20 percent. Below 10 percent, the annealingpoint is too low and above 20'percent, the liquidus is too high. CaOshould be present between about 5 and 25 percent. Below 5 percent, theglass melts poorly, above 25 percent, the expansion is too high. MgOshould be between about 0-12 percent. Above 12 percent, the liquidustemperature is too high. B 0 should be in the range of about 0-10percent. Above l0 percent, the chemical durability of the glass is poor.Optionally, a total of up to about 12 percent of the following oxidesmay also be incorporated to slightly modify the properties of the skinglass to within the desired ranges: BaO, SrO, ZnO, and La O Furthermore,up to about 5 percent of the following oxides can, also, be introducedto adjust the properties to within the previously stated limitations: LiO, Na O, K 0, Tio and Zr0,. When the laminate of this invention is usedto form tableware, the chemical durability of the skin glass is a veryimportant factor. Glasses for the adhered plies having a compositionwhich falls within the ranges described above have sufficient chemicaldurability so that they may be used as tableware.

A preferred glass for the adhered plies which falls within the abovecomposition range is, in weight percent on the oxide basis as calculatedfrom the batch, 59.4% 810,, 14.9% A1 14.6% CaO, 6.6% MgO, and 4.5% B 0This glass was made by preparing a batch comprising the following batchingredients:

Morgan 200 Mesh Sand 892.93 grams A-l Calcined Alumina 227.20 gramsBoric Acid 120.21 grams Calcium carbonate 391.40 grams Magnesium Oxide98.10 grarns This batch was then melted in a platinum crucible at l,550C. for 4 hours, cast into a slab and annealed at 725 C. The propertiesof the glass prepared in this manner were as follows: softening point90lC., annealing point 713 C., strain point 674 C., density of 2.570 g/cc.,viscosity at 1,300 C. of 800 poises, the coefficient of thermalexpansion between 0300 C. was 46.9 X l0"/ C., liquidus 1,176 C., andchemical durability (measured as weight loss in 5% HCl at 95 C. for 24hours) 0.21 mglcm Other glasses falling within the range of desiredproperties are disclosed in Table l.

TAB LE I.-ADHE RED present from between about 5 to percent by weight.Preferably, Na O should comprise over about one-half of the totalalkali, but not necessarily. The K 0 can be present in amounts of up toabout 6 percent. Li O is of little effect but may be incorporated insome glasses. If the total alkali metal oxide is less than 5 percent,the expansion is too low and the viscosity may be too high. On the otherhand, if the alkali is above 25 percent, the viscosity may be too'low.Optionally,

.from about 0 to 20 percent of the alkaline earth metal oxides may beadded. MgO and CaO are preferred. These oxides are added to adjust theproperties of the glasses to within the desired ranges. If more than 20percent is added, a satisfactory combination of expansion and viscosityis difficult to obtain. Additionally, up to a total of about 10 percentof the following oxides may be added to modify the properties: [a 0 TiOZrO,, Nb,0,,, ZnO, CdO, 6e0 PbO, Bi O CeO and B 0 To aid in fining, upto a total of about 2 percent of As,O and Sb,O, may be added. Similarly,up to about 1.5 percent chloride may be added to aid in fining. NaCl,KC], or CaCl are typical chloride fining agents which may be used. If itis desired to color the glass, up to 5 percent of the oxides of theLAYER GLASSES Weight percent oxides:

SiOz

LizO

Properties:

Expansion (0300 C.), (10- C Softening point, C Annealing point C. Strainpoint, Density, g./c0 Liquidus, 0..

The core glasses may either be clear, opacified, or thermallycrystallizable. We have found that our opal glasses can eitherspontaneously opacify during cooling or be opacified by a sub sequentheat treatment. The heat treated opals can also, by a particularsubsequent heat treatment, be converted to glassceramic materials.

We prefer to use clear core glasses of the alkali alumino-silicate typein the preferred embodiment of our invention. Generally speaking, thealumina (A1 0 maintains a relatively high viscosity at the laminatingtemperatures while allowing the addition of alkali metal oxides toobtain a sufficiently high coefficient of thermal expansion. The Si0should be present between about 50 and 75 percent by weight. Below 50percent, the viscosity at the forming temperatures is too low and theliquidus too high. Above 75 percent, the expansion is too low andmelting and forming becomes difficult. The Al O should be present in therange of about 10 to percent by weight. Below 10 percent, the viscosityis too low for forming and the expansion may also be too low. Above 30percent,

ing batch ingredients:

Keystone No. 1 Dry Sand 62.4 grams Nepheline Syenite 880.0 grams SodaAsh 31.6 grams Sodium Nitrate 54.9 grams Dolomite Limestone 96.0 gramsArsenic Trioxide 10.0 grams The properties of this glass were asfollows: softening point 863 C., annealing point 633 C., strain point588 C., liquidus temperature of 1,1 14 C., the coeflicient of thermalexpansion between 0-300 C. was 92.1 X 10'/' C., density 2.484 g/cc, ndthe viscosity at l,300 C. was 4500 poises.

Other clear alkali aluminosilicate core glasses which have the liquidusis too high. The total alkali metal oxide Sllfllilg properties thedesired ranges are disclosed in Table II.

TABLE IL-CLEAR ALKALI ALUMINOSILICATE CORE GLASSES Properties:

Expansion (0300 0.), X10' C Softening point, C.

Annealln point, C

in s, Viscosity at 1300 C., poises Although we prefer to use clear coreglasses of the alkali aluminosilicate type, clear core glasses of thealkaline earth metal aluminosilicate type may be substituted as afunctional equivalent. Thus, the alkaline earth metal aluminosilicateglasses may be melted, shaped, formed, and laminated with the samefacility as the alkali aluminosilicate glasses. However, formanufacturing reasons we prefer to use the alkali aluminosilicateglasses. Normally, the alkaline earth metal aluminosilicate glassescontain little, if any, of the alkali metal oxides. The SiO should bebetween 40 and 60 percent by weight. Below 40 percent, the glass is toofluid. Above 60 percent, the expansion is too low. The A1 should bepresent in the range of about to percent by weight. Below 5 percent,

it is difficult to adjust the expansion, viscosity, and liquidus 15viscosity of the glass. Above 15 percent, the liquidus is too high. BaOshould be present between and 50 percent by weight. Below 20 percent,the expansion is too low and above 50 percent, the glass is too fluid.The SrO may be present up to percent since above 25 percent, it isdifficult to adjust the 20 expansion and viscosity. Up to a total of 10percent of the following oxides may be incorporated to obtain specificproperties: La O B 0 CaO, MgO, TiO ZrO PbO, ZnO, CdO, and P 0 Apreferred clear alkaline earth metal aluminosilicate core 25 glass whichfalls within the above composition range is, in weight percent on theoxide basis as calculated from the batch 45.9% SiO 9.1% A1 0 38.5% BaO,and 6.5% SrO. This core glass was made from a batch comprising thefollowing batch The properties of this glass were as follows: expansion70.7 X l0' /AL C., softening point 903 C., annealing point 719 C.,strain point 676 C., density 3.372 g/cc, liquidus 1142, and viscosity at1300 C. was 1,400 poises.

Other clear alkaline earth metal aluminosilicate core glasses 4 whichmeet the aforementioned properties are disclosed in Table 111.

TABLE TIL-CLEAR ALKALINE EARTH METAL ALUMINO- SILICATE CORE GLASSESProperties:

Expansion (0300 0.),

Softening point, C. Annealing point 0. Strain point, d.

It is desirable to produce opaque, or opal, bodies and there are severalglass systems which will yield opal glasses; however, we prefer to usethe fluoride opal system for core glasses 10 of our invention. Thispreference is based upon the ease of manufacturing the laminate.

We have been able to produce such an opal by slightly modifying theclear alkali aluminosilicate core glass compositions and makingadditions of from 3-8 percent fluoride. The modifications consist of:lowering the A1 0 range to 3-20 from 10-30 percent, lowering the totalalkali metal oxide range to 3-20 from 5-25 percent, lowering the minimumamount of Na,O to one-third of the total alkali from one-half of thetotal alkali, and raising the maximum K 0 to 8 from 6 percent. Theeffect of any constituent being outside the range for the opal glass isthe same as for that constituent being outside the range in the clearalkali aluminosilicate core glass. it the fluoride is below 3 percent,the opal will not be sufliciently dense and above 8 percent theviscosity of the glass is too low. An important property of these opalsis that they opalize spontaneously during cooling and hence do notrequire a subsequent heat treatment for opalization. This opalization isdue to the separation of calcium fluoride crystals in the glass. Thefluoride can be introduced into the batch in compounds such as CaF NaSiF AlF or Na AlF A dense white spontaneous opal which falls within theabove composition range is, in weight percent as calculated from thebatch, 65.1% SiO 6.1% A1 0 5.0% Na O, 1.9% K 0, 15.8% CaO, and 6.1% F.Such a glass was made from a batch comprising the following ingredients:

Keystone No. 1 Dry Sand 6532.0 grams Soda Ash 662.0 grams Sodium Nitrate300.0 grams Calcined Alumina 619.0 grams Calcium Carbonate 1 192.0 gramsFluorspar 1271.0 grams Potassium Carbonate 279.0 grams The properties ofthe glass prepared from the above batch were as follows: softening pointgreater than 970 C., annealing point 655 C., strain point 612 C.,liquidus temperature of 1,194 C., coefficient of thermal expansion 79.1X 10'/ C., density 2.471 g/cc, and the viscosity at 1300" C. was 600poises.

A dense gray spontaneous opal which falls within the above compositionrange is, in weight percent as calculated from the batch, 62.8% SiO l1.8% A1 0 4.03% Na O, 3.38% K 0, 0.35% MgO, 12.7% CaO, 0.54% Fe O and4.4% F. This glass was made from a batch comprising the followingingredients:

Keystone No. 1 Dry Sand 103.3 grams St. Lawrence Fluorspar Tailings371.1 grams C-20 Feldspar 558.8 grams A typical analysis, by weight, ofthe St. Lawrence Fluorspar Tailings is as follows: 41.8% SiO 1 1.4% F,10% Na O, 31.0% CaO, 0.4% MnO 1.3% Fe O 3.4% A1 0 0.1% MgO, 1.2% K 0,and 8.0% C0 The properties of the glass prepared from the above batchwere as follows: annealing point 721 C., strain point 671 C., liquidustemperature of 1,170 C., coefficient of thermal expansion 79.5 X 10"/C.,density 2.484 g/cc, and the viscosity at 1,300 C. was 1,000 poises. 7

Other spontaneous opals which have properties within the desired rangesare disclosed in Table IV.

TABLE IV.-SPONTANEOUS OPAL CORE GLASSES Properties:

Expansion (0300 C.) X10" C 69.8 59.1 71 5 73 8 68.3 79 1 67 6 72 5 84 293. 7 Softening point, C.... 970 901 -843 Annealing point 737 G54 G22Strain point, L 694 600 569 2. 443 2. 523 2. 511

Density, g./cc

Another type of opal which can be used as a core glass is known as aheat-treatable opal. A heat-treatable opal is one in which opalizationis developed by a subsequent nucleation and growth heat treatment andone which does not opalize during cooling from the liquidus temperature.One such opal major crystalline phase is zinc orthosilicate. The totalcrystalline content of the resultant opal glass is less than 10 percentby volume of the glass.

Other heat-treatable opals which have properties within the 5 .desiredranges are disclosed in Table V.

TABLE V.FLUORIDE-NUCLEATED ZINC ORTHOSILICATE-TYPE OPAL GLASSES Weightpercent oxides:

Properties:

Softening point,

Annealing point, Strain point, C Density, gJcc... Liquidus, C

has as its major crystalline phase zinc orthosilicate crystals whichhave grown on fluoride nucleation sites. However, the opal is stillknown as a fluoride opal. The composition range for a heat-treatableopal core glass of this type is set forth hereinafter in weight percentas calculated from the batch. The SiO should be between about 50 and 70percent. Below 50 percent, the viscosity is too low. If the SiO is above70 percent, the viscosity is too high. A1 0 should be between about andpercent. Above 25 percent, the viscosity is too high and the glass tendsto have a low opacity when heat treated at low temperatures. Below 15percent, the viscosity is too low and again the glass may have a lowopacity. Na O should be between about 7 and 14 percent. Below 7 percent,the coefficient of thermal expansion is too low and above 14 percent,the viscosity is too low. The ZnO should be between about 5 and 12percent. If it is below 5 percent, the glass will have a low opacity andif it is above 12 percent, the viscosity will be too low. Fluorineshould be between about 2.5 and 7 percent. Below 2.5 percent, theopacity is too low and above 7 percent, the viscosity is too lowfl'hetotal amount of impurities should not exceed about 3 percent since theopacity may be low and problems relating to the glasss past thermaltreatment may exist. These impurities include As,O CaO, MgO, B 0 Li,O,and BaO. Minor additions of MnO,, Fe O and other known colorants may beused to color the core compositions. The range of heat treatment,schedules for these opals comprise: nucleation between about 500 C. to650 C. for at least 10 minutes and growth above about 650 C. for atleast 10 minutes.

A preferred heat-treatable opal core glass composition which is withinthe previously discussed ranges is as follows: 58.3% SiO,, 18.4% A1 0,,10.2% Na O, 8.3% ZnO, 3.9% F, 0.5% CaO, 0.2% MgO, and 0.15% 8,0,. Thisglass was made from a batch comprising the following:

Berkeley fine dry special sand 562.0 grams A-l Calcined Alumina 186.0grams Na,CO, 110.0 grams NaNO 27.0 grams Na SiF 83.0 grams 100 GranuleZnO 83.0 grams CaCO, 9.0 grams Calcined Magnesite 2.0 grams Anhydrous8,0,, 1.5 grams The preferred glass, as formed, is clear and has thefollowing properties: liquidus temperature of 1,213, annealing point 540C., strain point 494 C., the coefficient of thermal expansion between0300 C. is 65.1 X 10"/ C., the density is 2.500 g/cc, and the viscosityat 1300 C. is 2400 poises. This glass was nucleated at between 540-640C. for k hour. Opalization took place upon further heating at 720 C. for1% hour. This treatment results in a dense, white opal where the Slightmodifications of the heat treatable opal glass composition ranges willyield thermally-crystallizable glasses. These glasses can be convertedto glass-ceramics by first opalizing the glass and then subjecting theopalized glass to a further heat treatment. The major glass-ceramicphase, nepheline, probably nucleates upon the previously formed zincorthosilicate crystals. Thus, this glass-ceramic may be characterized asa zinc-orthosilicate-nucleated nepheline-type glass-ceramic. In order toform the glass-ceramic, the opal must be heat treated at temperaturesabove those at which the opal was formed. These glass-ceramics haveapproximately the following composition in weight percent on the oxidebasis as calculated from the batch: SiO, from about 44 to 61%, A1 0 fromabout 19 to 23%, Na O from about 10 to 14%, ZnO from about 7 to 10%, andF from about 3 to 6%. If the composition is outside the aforementionedranges, a glassceramic cannot be formed. However, if the composition isstill within the previously mentioned heat-treatable opal range an opalmay be formed. On the other hand, if the composition is also outside theheat-treatable opal range, the associated effect is that which has beenpreviously described. The impurity level is about the same as requiredfor the heat-treatable opal. The heat treatment to form theglass-ceramic comprises opalizing the glass and then heating theopalized glass to a temperature between 750 C. and 850 C. for asufficient length of time to form the glass-ceramic. A preferredglasscerarnic which falls into the above composition range is asfollows: 54.0% SiO,, 21.2% A1,0,, 13.1% Na,0, 8.0% ZnO, 3.1% F, and 0.6%CaO. This glass-ceramic was made from the following batch ingredients:

Berkeley fine dry special sand 518.9 grams A-l Calcined Alumina 214.3grams Soda Ash 171.3 grams Sodium Nitrate 26.8 grams Zinc Oxide 89.3grams Sodium Silicofluoride 64.9 grams Calcium Carbonate 10.8 grams 100X l0"/ C. and containing nepheline as the principal crystal phase.

Other glass-ceramics formed from the heat-treatable opals which meet theaforementioned properties are disclosed qw in able V TABLE VIZinc-Orthosilicate-Nucleated Nepheline-Type Glass- Ceramics SiO 55.152.9 52.9 50.9 A1 21.7 20.8 20.8 19.9 Na o 10.6 l3.1 13.1 12.6 ZnO 9.18.7 8.8 8.4 F 3.0 3.8 3.8 5.5 CaO .5 1.03 MgO .75 B 0 .75 A3 0 0.5 .5Expansion Glass (0300C.), l0"/C. 78.9 80 77.4 75.4 Softening Point, C.776 750 Annealing Point, C. 556 548 570 540 Strain Point, C. 510 508 524502 Density, g/cc 2.557 2.552 2.562 2.565 Liquidus, C. 1105 1197 12451171 Viscosity at 1300C., Poises 1900 1200 1000 750 ExpansionGlass-Ceramic (0 300C.), X-/C. 100 101 102 102 In addition to using azinc-orthosilicatemucleated nepheline type glass-ceramic as the corematerial, it is possible to use titania-nucleated nepheline-typeglass-ceramics. The thermally crystallizable glass having theappropriate composition must be within the viscosity and temperaturelimitations for forming the laminated sheet. Furthermore, it must havean appropriate coefficient of thermal expansion; the glass must also becapable of being heat treated at temperatures where the shaped body willnot distort nor the adhered plies deform The occur. Below 9% fia O,fine-grained crystallization will not occur below 850 C. lfthe CaOexceeds 9 percent, no crystallization will occur below 850 C. MgO shouldbe between about 0.25 and 3 percent. Below 0.25 percent, nucleationcannot take place and above 3 percent, the viscosity of the glass is toolow. TiO is used as the nucleating agent and is present in the range ofabout 3.0 to 6.0 percent. In addition to the above constituents, up to atotal of about 5 percent of impurities such as CdO, ZnO, Ago K 0, and B0 may be present in the glass. The range of heat treatments, orceramming schedules, to form the above-ceramic comprises heating thethermally crystallizable glass at temperatures between 700 C. I and 750C. for at least 10 minutes so as to nucleate the crystal 7 phase andthen heating the nucleated glass at temperatures between 750 C. and 850C. for at least 10 minutes so as to cause the nucleated crystals togrow. A preferred glass-ceramic which is within the above compositionrange is as follows:

52.15% SiO 26.15% A1 0 10.30% Na O, 6.60% CaO,

0.95% MgO, 3.00% TiO 0.35% A5203, and 0.50% Li,O. This glass-ceramic wasmade from the following batch ingredients:

Berkeley fine dry special sand 431.4 grams A-l Calcined Alumina 244.6grams Soda Ash 156.3 grams Sodium Nitrate 27.5 grams Lime Hydrate 90.1grams Magnesium Oxide 9.1 grams Titania 30.2 grams Arsenic Trioxide 5.0grams Petalite 1 13.6 grams The glass as formed was clear but wasconverted to a glassceramic material when heat treated according to thefollowing schedule: heat to 700 C. at a rate of 300 C./hour, then heatto 825 C. at a rate of 30 C./h0ur, next hold at 825 C. for 1 hour, andthen cool to room temperature. The resultant glassceramic was gray-whitematerial having a coefiicient of thermal expansion of 97 X 10"/ C.

Other titania-nucleated nepheline-type glass-ceramics which meet theaforementioned properties are disclosed in Table VII.

TABLE VII.-TITANIA-NUCLEATED NEPHELINE-TYPE GLASS-CERAMICS Weightpercent oxides:

SiOz

LizO Properties:

Expansion glass (0300 C-), Xl0' C 69.6 78.1 72, 7 Softening point, C 909884 903 894 905 Annealing point, C. 681 677 661 703 627 690 703 Strainpoint, C 640 634 622 659 593 652 663 Density, g./cc 2. 459 2. 533 2. 5172. 540 2. 5 2. 52 2. 527 Liquidus, C 1,073 1, 233 1,192 1,245 1,0901,143 1, 243 Expansion glass-ceramic 77. 3 72. 6 90. 8 94. 8 90 105.199. 3

composition range of a glass-ceramic which will fulfill the A finalconsideration in selecting the compositions is that afore-mentionedrequirements is hereinafter described. The SiO should be between about50 and percent by weight. Above 65 percent, the glass cannot beconverted to a glassceramic below 850 C. Below 50 percent, the glass istoo fluid at high temperatures. Al,0 should be between about 20 and 30percent by weight. Above 30 percent, the liquidus temperature is toohigh and no crystallization will occur below 850 C. Below 20 percent Al,O,,, crystallization will not occur. The total of Na o and CaO must bebetween about 15 and 20 percent. Above 20 percent, less desirable coarsegrained crystallization will occur below 850 C. Below 15 percent, no fmegrain crystallization will occur below 850 C. Individually, the N320should be between about 9 and 20 percent and the CaO between about 0 and9 percent with the total being between 15 and 20 percent. If the Na,0were above 20 ILEBEEQW'K i -fr tal z an 329 5 the forming processproduces substantial amounts of waste, or cullet, which should berecycled into the core glass batch since the cullet is primarily a coreglass. However, adjustments must be made in the core glass compositionsso that the constituents which are present in the adhered plies and notin the core glass will not deleteriously afiect the properties of thecore glass. These constituents will be relatively minor since the amountof glass from the adhered plies being recycled into the core glass batchis relatively small.

The following examples will be better illustrate the laminates of ourinvention:

EXAMPLE 1 Two separate sheets of glass for the adhered plies of thefollowing composition were formed: 57.77% Si0 14.94% A1 0 996% sas-357M110. 39. B293 esame a 050% AS203. A single sheet of clear alkalialuminosilicate core glass of the following composition was also formed:56.84% SiO 19.80% A1 12.80% Na O, 3.18% CaO, 4.30% K 0, 2.1 1% MgO, and0.99% AS203. These sheets were fused together, at

about 1,300 C., so as to form a three-ply laminated sheet wherein thecore glass was the center ply. At 1,300" O, the viscosity of the coreglass was about 4,000 poises while that of the adhered plies was about1000 poises. Thus, the viscosity ratio, at the laminating temperature,was 4:1. The liquidus temperature of the core and adhered plies were1047 C. and 1,144 C., respectively. The hot laminated sheet was nextsagged into a custard cup shaped mold, trimmed, removed from the mold,and allowed to cool. The resultant custard cut had a rim diameter of 41% inches, a bottom diameter of 3 inches, and was 1 96 inches high. Thecore glass ply was 0.080 inch thick and the total thickness of theadhered plies was 0.004 inch, resulting in a thickness ratio of about20: 1. At the laminating temperature, the coefficient of thermalexpansion of the core glass was 94 X 10"/ C. while that of the adheredplies was 46 X l0' C. This combination of thickness ratios and expansionmismatch resulted in a body exhibiting a MOR of about 48,000 psi. Thecustard cup was found to withstand impacts of up to 0.41 foot pounds. Todetermine violence of breakage, a center punch test was used. This testcomprises placing a center punch in the center of the cup and strikingthe punch with increasing force until the cup breaks. Upon testingseveral cups, it was found that they break into to 10 pieces withlittle, if any, explosive violence.

EXAMPLE 11 Glass for the adhered plies of the following composition wasmelted: 56.70% SiO 14.85% A1 0 11.92% CaO, 8.57% MgO, and 7.90% B 0 Aspontaneous opal glass of the following composition was also melted:58.57% SiO 13.46% A1 0 5.35% Na O, 4.30% K' O, 11.39% CaO, .82% MgO,0.71%

B 0 and 5.40% F. These glasses were then formed into a three plylaminate, as in Example 1, wherein the opal glass was the center ply.These glasses were laminated at l,285 C. where the viscosity of the-coreglass was about 500 poises and the viscosity of the adhered plies wasabout 470 poises. Thus, the viscosity ratio was about 1:1. This laminatewas then formed into a custard cup as in Example 1. In this cup thethickness of the core was 0.080 inch while the total thickness of theskin glass was 0.004 inch. These thicknesses resulted in a thicknessratio of about 20: 1. The coefficient of thermal expansion at thelaminating temperature of the core glass was 89 X l0"/ C. while that ofthe skin glass as 47 X l0' C. This combination of thickness ratios andexpansion mismatches resulted in a body exhibiting a MOR of about 47,000psi. The

cup was then impact tested and found to withstand impacts of up to 0.04foot-pounds. When tested for violence of breakage the cups were found tobreak into about 1 1 pieces with little, if any, explosive violence.

EXAMPLE III A heat-treatable opal core glass of the followingcomposition was melted: 59.80% SiO 18.35% A1 0 10.80% Na O, 1.05% CaO,0.40% MgO, 7.40% Zno, 3.80% F, and 0.35% B 0 This glass was laminated asin Example I with two sheets of glass for the adhered plies as inExample 11. The laminating temperature was 1,280 C. whereat theviscosity of the core glass was about 2,200 poises and that of theadhered plies was about 470 poises. Thus, the viscosity ratio was about5:1. The

liquidus of the adhered plies was about l,l26 C. while that of the coreglass was 1 ,166 C. A custard cup was then formed as in Example 1wherein the core thickness was 0.090 inch and the total thickness of theadhered plies was 0.006 inch. After forming, the cup was opalized by aheat-treatment according to the following schedule: heat to 630 C. andhold thereat for 1% hour, next heat from 630 C. to 710 C. and holdthereat for hour, and then cool to ambient temperature. The particularthicknesses resulted in a ratio of about 15: l. The coefficient ofexpansion at the laminating temperature of the core glass was 70 X l0"'/C. This combination of thickness ratio and expansion mismatch resultedin a body exhibiting a MOR of about 33,000 psi. The impact strength ofthe body was about 0.5 foot-pounds. The violence of breakage was quitelow in that the bodies tested broke into from 3 to 5 pieces.

EXAMPLE IV A thermally crystallizable core glass of the followingcomposition was melted: 54.60% SiO,, 21.20% A1 0 13.10% Na O, 0.60% CaO,8.60% ZnO, and 3.10% F. This glass was laminated, as in Example I, withtwo sheets of glass for the adhered plies as in Example II. Thelaminating temperature was 1,300 C. and at that temperature theviscosity of the core glass was about 1,000 poises and that of theadhered plies was about 470 poises. Thus, the viscosity ratio was about2: 1. The liquidus of the adhered plies was at about 1,126 C. while thatof the core glass was about 1,197 C. A custard cup was then formed as inExample I wherein the core thickness was 0.090 inch and the totalthickness of the adhered plies was 0.006 inch. the cup was clear and thecore glass had an expansion of X 10"/ C. After forming, the cup wascerammed by a heat treatment according to the following schedule: heatto 630 C. and hold thereat for k hour, heat to 750 C. and hold thereatfor 1 hour, and then cool to ambient temperature. This treatmentconverted the glass core to a zinc orthosilicate-nucleatednepheline-type glass-ceramic. The expansion of the glassceramic was nowX 10"/ C. The particular thicknesses resulted in a ratio of about 15:1.This combination of thickness ratio and expansion mismatch resulted in abody exhibiting a MOR of about 50,000 psi. The impact strength of thesecustard cups was about 0.5 foot-pounds. When the custard cups weresubjected to the center punch test, they broke into from 10 to 50pieces.

EXAMPLE V A thermally crystallizable core glass of the followingcomposition was melted: 52.15% SiO 26.15% A1 0 10.30% Na O, 6.60% CaO,3.00% TiO 0.95% MgO, 0.35% A5 0 and 0.50% Li O. A glass for the adheredplies of the following composition was also melted: 62.2% SiO 14.8% A1 0and 23.0% CaO. These glasses were laminated as in Example 1, at aboutl,300 C. At that temperature, the viscosity of the core glass was about2,800 poises and that of the adhered plies was about 1400. Thus, theviscosity ratio was about 2:1. The liquidus of the adhered plies wasabout 1,139 C. while that of the core glass was about l,224 C. A custardcup was then formed, as in Example 1 wherein the core thickness was0.100 inch and the total skin thickness was 0.005 inch. The cup wasclear and the clear uncerammed core had an expansion of 70 X 10"/ C.After forming, the cup was cerammed by a heat treatment according to thefollowing schedule: heat to 700 C. and hold thereat for ye hour, nextheat at the rate of 100 C./hour to 810 C. and hold thereat for at least1% hour, and then cool to ambient temperature. This treatment convertedthe glass core to a titania-nucleated nepheline-type glasscerarnic core.The expansion of the glass-ceramic core was now 97 X l0"'/ C. while thatof the adhered plies was 54 X 10 /AL C. The particular thicknesses werein the ratio of about 20:1. This combination of thicknesses andexpansion mismatch resulted in a body exhibiting a MOR of about 40,000psi. The impact strength of these custard cups was about 0.35foot-pounds. When the custard cups were subjected to the center punchtest, they broke into two pieces.

Although the above examples are set forth only for five specificcombinations of glasses, many other combinations may be used. Forexample, various combinations of glasses, from Table l, and glasses,from Tables 11, 111, IV, V, V1, and VII can be laminated so as to form astrengthened article, provided that the forming and physical parametersare maintained within the aforementioned limits. Furthermore, glassesother than those disclosed in Tables 1, 11, 111, IV, V, VI, and V1] maybe selected provide they have properties within those limitations setforth for the various glasses. Thus, although there are many examples ofspecific glasses and combinations thereof, many other glasses andcombinations may be used provided their compositions and properties arewithin the ap plicable limitations.

The above discussion and descriptions have related to all glass andmixed glass and glass-ceramic laminates. We have also discovered thatall-glass-ceramic laminates can be prepared. These laminates areprepared by laminating sheets of thermally crystallizable glasses andthen heat treating the laminateso as to convert the glass to aglass-ceramic. The forming parameters such as viscosity ratios, liquidustemperature, etc. are the same for the thermally crystallizable glassesas they are for the other systems described. The heat treatingparameters are also similar in that there should be a reversal of theviscosity relationship. However, the maximum heat treating temperaturesare greater than 850 C. Furthermore, the thicknesses and ratios for theglass containing laminates also apply to the all-glass-ceramiclaminates. As in the all-glass and mixed-glass and glass-ceramiclaminate, the strength is related to the strain which could beapproximated by the expansion mismatch at the setting point of thesoftest glass in the laminate. However, in the case of anall-glass-ceramic laminate, the setting point of the softestglass-ceramic is several hundred degrees greater than that of a glass.Thus, the expansion mismatch at the setting temperature can be less eventhough the strain is the same, since the setting point to usetemperature differential is greater for an all-glass-ceramic laminatethan for a laminate having glass therein. Normally, in theall-glass-ceramic laminate, the expansion mismatch will be at least 15 X10"/ C. which is the same as that for a laminate having glass therein.However, in some cases the expansion mismatch may be as low as 5 X 10'/C. Furthermore, useful glass-ceramic laminates can be prepared whichhave very high or very low coefficients of thermal expansions. Thus, theglassceramic laminates may fall outside those expansion ranges which wehave discovered for glass containing laminated systems.

EXAMPLE VI A laminate was prepared wherein the core ply had thefollowing composition, in weight percent on the oxide basis: 64.8% SiO20.0% A1 2.0% B 0 0.5% Na O, 0.2% K 0, 3.5% Li O, 1.8% MgO, 2.2% ZnO,4.25% TiO and 0.75% As O The adhered plies had the followingcomposition, as calculated from the batch, in weight percent on theoxide basis: 64.5% SiO 22.9% A1 0 0.3% Na O, 0.2% K 0, 1.8% MgO, 1.5%ZnO, 3.8% Li O, 1.0% A5 0 2.0% TiO and 2.0% ZrO After lamination andheat treatment, the core ply had as its principal crystal phasesbeta-spodumene solid solu tion and rutile. The adhered plies had astheir principal crystal phase beta-quartz solid solution. Thecoefficient of expansion of the glass-ceramic core ply was X 10 /C.while that of the adhered plies was 5 X l0' C. These expansions resultedin a mismatch of X l0 C.

EXAMPLE Vll A laminate can be prepared where both the core ply and theadhered plies are nepheline-type glass-ceramics. A thermallycrystallizable glass sheet of the following composition, in weightpercent on the oxide basis, was prepared for the core ply: 40.5% SiO31.2% A1 0 10.4% Na- O, 9.5% K 0, 0.2% CaO, 0.1% MgO, 7.4% TiO and 0.7%AS203. Two sheets of the following thermally crystallizable glasscomposition, in weight percent on the oxide basis, were prepared for theadhered plies: 43.5% $0,, 31.5% A1 0 12.5% BaO, 12.5% Na O, and 6.0% TiOThese sheets can be laminated and then heat treated to form theglass-ceramic laminate. After heat treatment, the core ply would have asits principal crystal phases anatase and nepheline solid solution with acorresponding coefiicient of thermal expansion of 130 X l0 C. Theadhered plies have as their principal crystal phases betaspodumene solidsolution and rutile with a corresponding coefficient of thermalexpansion of 85 X 10 C. This would result in an expansion mismatch ofabout 45 X 10"/ C.

3 EXAMPLE VIII Another laminate can be prepared wherein the core ply isof the same composition as the adhered plies described in din ExampleVII and wherein the present adhered plies have the followingcomposition, in weight percent on the oxide basis, 56.2% SiO 19.8% A1 014.5% MgO, 9.1% TiO,, and 0.4% As O After lamination and heat treatment,the adhered plies had as their principal crystal phase cordierite. Theexpansion of the adhered plies was 54 X lO C. while that of the core wasX 10/ C. This resulted in a mismatch of 31 X l0"'/ C.

EXAMPLE IX A laminate can also be prepared wherein the core ply has thesame composition as in Example VII and the adhered plies have the samecomposition as the core ply of Example VI. After lamination and heattreatment, the adhered plies had as their principal crystal phasesbeta-spodumene solid solution and rutile. The expansion of the core wasthen X 10"'/ C. while that of the adhered plies was 1 l X 10 C.

EXAMPLE X A laminate can be prepared wherein the core ply is of the samecomposition as the adhered plies in Example 1X. The adhered plies forthis laminate have the following composition, in weight percent on theoxide basis, 50.1% SiO 35.8% A1 0 8.4% Li O, 4.7% TiO 0.1% Na O, 0.2% K0, 0.5% Fe o and total CaO and MgO is 0.2%. After lamination and heattreatment, the adhered plies had as their principal crystal phasebeta-eucryptite solid solution with an expansion of 2 x l0"/AL C. Thisresulted in a mismatch of 9 X 10'-/ C.

Thus, our invention also includes the discovery of all-glassceramiclaminates and the method of manufacturing said laminates.

We claim:

1. A high strength glass or glass and glass-ceramic laminate comprisinga tensilely-stressed core portion and a compressively stressed surfacelayer fused to and substantially enveloping said core portion such thatthe fused glass surface between said core portion and said surface layeris essentially defect-free wherein:

a. the ratio of the thickness of said core portion to the totalthickness of said surface layer is at least about 1011;

b. the thickness of said surface layer is at least about 0.002

inch; and

c. the coefiicient of thermal expansion of said surface layer at thesetting point is at least about 15 X 10' C. less than the coefiicient ofthermal expansion of said core portion.

2. A high strength laminate according to claim 1 wherein a. thecoefficient of thermal expansion of the compressively stressed adheredlayer is between 3080 X 10"/ C.; and

b. the coefficient of thermal expansion of the tensilely stressed coreportion is between 60-1 10 X l0"/ C.

3. A high strength laminate according to claim 1 consisting of athree-ply sheet having a core ply and two adhered plies.

4. A high strength laminate according to claim 3 wherein the ratio ofthe thickness of the tensilely-stressed ply to the total thickness ofthe compressively-stressed plies adjacent thereto is less than 30: 1.

5. A high strength laminate according to claim 3 wherein the adheredplies are glass and consist essentially of, in weight percent on theoxide basis as calculated from the batch:

a. from 50 to 65% 810 b. from 10 to 20% A1 0 c. from 5 to 25% CaO;

d. from 0 to 12% MgO;

e. from 0 to 10% B 0 f. from 0 to 12% total of at least one compoundselected from the group consisting of BaO, SrO, ZnO, and 1.3 0,; and

(g) from O to 5 percent total of at least one compound selected from thegroup consisting of Li O, Na O, K 0, TiO and 2:0,.

6. A high strength laminate according to claim 3 wherein.

the core ply is a clear alkaline earth metal aluminosilicate glass,consisting essentially of, in weight percent on the oxide basis ascalculated from the batch: a. from 40 to 60% SiO,; b. from 5 to 15% A1c. from 20 to 50% BaO; d. from O to 25% SrO; and e. from 0 to 10% total,of at least one compound selected from the group consisting of [a 0 B 0CaO, MgO, TiO ZrO,, PbO, ZnO, CdO, and P 0 7. A high strength laminateaccording to claim 3 wherein the core ply is a clear alkalialuminosilicate glass consisting essentially of, in weight percent onthe oxide basis as calculated from the batch:

a from 50 to 75% SiO b from to 30% A1 0 0 from 5 to 25 percent totalalkali oxide with Na O comprising at least one-half of the totalalkalimetal oxide and with from 0 to 6% K 0;

d from 0 to 20 percent total alkaline earth metal oxide;

e from 0 to l0 percent total of at least one compound selected from thegroup consisting of La o TiO ZrO Nb O ZnO, CdO, 6e0 PbO, Bi 0 CeO and B0 f. from 0 to 2 percent of at least one compound selected from thegroup consisting of A5 0 and Sb O g from 0 to 1.5 percent chloride; and

h from 0 to 5 percent total of at least one coloring oxide selected fromthe group consisting of the oxides of Cr, Mn, Fe, Co, Cu, Nd, V, and Ni.

8. A high strength laminate according to claim 3 wherein the core ply isa spontaneous opal glass consisting essentially of, in weight percent onthe oxide basis as calculated from the batch:

a from 50 to 75% SiO b from 3 to 20% Al O c from 3 to 20% total alkalimetal oxide with Na O compris ing from at least one-third of the totalalkali metal oxide and with from 0 to 8% K 0;

d from 0 to 20 percent total alkaline earth metal oxide;

e. from 0 to 10 percent total of at least one compound selected from thegroup consisting of La 0 TiO ZrO Nb O ZnO, CdO, 0e0 PbO, Bi O CeO and B0 f. from 0 to 2 percent of at least one compound selected from thegroup consisting of AS203 and Sb O g. from 0 to 1.5 percent chloride;

h. from O to 5 percent total of at least one compound selected from thegroup consisting of the oxides of Cr, Mn, Fe, Co, Cu, Nd, V, and Ni; and

i. from 3 to 8 percent fluoride.

9. A high strength laminate according to claim 3 wherein the core ply isa heat-treatable opal glass consisting essentially of, in weight percenton the oxide basis as calculated from the batch:

a. from 50 to 70% SiO b. from to 25% A1 0 c. from 7 to 14% Na O;

d. from 5 to 12% ZnO;

e. from 2.5 to 7 percent fluoride; and f. from 0 to 3 percent total ofat least one compound selected from the group consisting of A5203, CaO,MgO, B 0 U 0, and BaO. 10. A high strength laminate according to claim 3wherein the core ply is a zinc orthosilicate-nucleated nepheline-typeglass-ceramic consisting essentially of, in weight percent on the oxidebasis as calculated from the batch:

a. from 44 to 61% $0,;

b. from 19 to 23% A1 0 c. from 10 to 14% Na O;

d. from 7 to 10% ZnO;

e. from 3 to 6 percent fluoride; and

f. from 0 to 3 percent total of at least one compound selected from thegroup consisting of As O CaO, MgO,

B 0 U 0, and BaO.

11. A high strength laminate according to claim 3 wherein the core plyis a titania-nucleated, ne heline-type glass-ceramic consistingessentially of, in wei t percent on the oxide basis as calculated fromthe batch:

a. from 50 to 65% SiO b. from 20 to 30% A1 0 0. from 15 to 20 percenttotal Na O and CaO with 1) from 9 to 20% Na O, and (2) from 0 to 9%C210; d. from 0.25 to 3% MgO;

e. from 3 percent to 6 percent TiOg; and f. from O to 5 percent total ofat least one compound selected from the group consisting of CdO, ZnO, AsO K 0, and B 0 12. A high strength glass-ceramic laminate comprising atensilely-stressed surface layer fused to and substantially envelopingsaid core portion such that the fused surface between said core portionand said surface layer is essentially defectfree wherein:

a. the ratio of the thickness of said core portion to the totalthickness of said surface layer is at least about 10; l

b. the thickness of said surface layer is at least about 0.002

inch; and

c. the coefficient of thermal expansion of said surface layer is atleast about 5 X l0-'/ C. less than the coefficient of thermal expansionof said core portion.

13. A high strength laminate according to claim 12 consisting of athree-ply sheet having a core ply and two adhered plies.

14. A high strength laminate according to claim 13 wherein the ratio ofthe thickness of the tensilely stressed ply to the total thickness ofthe compressively stressed plies adjacent thereto is less than 30:1.

15. A high strength laminate according to claim 13 wherein:

a. the tensilely-stressed core ply is a glass-ceramic selected from thegroup consisting of beta-spodumene solid solution and rutile, anataseand nepheline solid solution; and

b. the compressively-stressed adhered plies are a glassceramic selectedfrom the group consisting of beta-quartz solid solution, beta-spodumenesolid solution, and rutile, cordierite and beta-eucryptite solidsolution.

2. A high strength laminate according to claim 1 wherein a. the coefficient of thermal expansion of the compressively stressed adhered layer is between 30-80 X 10 7/* C.; and b. the coefficient of thermal expansion of the tensilely stressed core portion is between 60-110 X 10 7/* C.
 3. A high strength laminate according to claim 1 consisting of a three-ply sheet having a core ply and two adhered plies.
 4. A high strength laminate according to claim 3 wherein the ratio of the thickness of the tensilely-stressed ply to the total thickness of the compressively-stressed plies adjacent thereto is less than 30:1.
 5. A high strength laminate according to claim 3 wherein the adhered plies are glass and consist essentially of, in weight percent on the oxide basis as calculated from the batch: a. from 50 to 65% SiO2; b. from 10 to 20% Al2O3; c. from 5 to 25% CaO; d. from 0 to 12% MgO; e. from 0 to 10% B2O3; f. from 0 to 12% total of at least one compound selected from the group consisting of BaO, SrO, ZnO, and La2O3; and (g) from 0 to 5 percent total of at least one compound selected from the group consisting of Li2O, Na2O, K2O, TiO2, and ZrO2.
 6. A high strength laminate according to claim 3 wherein the core ply is a clear alkaline earth metal aluminosilicate glass, consisting essentially of, in weight percent on the oxide basis as calculated from the batch: a. from 40 to 60% SiO2; b. from 5 to 15% Al2O3; c. from 20 to 50% BaO; d. from 0 to 25% SrO; and e. from 0 to 10% total, of at least one compound selected from the group consisting of La2O3, B2O3, CaO, MgO, TiO2, ZrO2, PbO, ZnO, CdO, and P2O5.
 7. A high strength laminate according to claim 3 wherein the core ply is a clear alkali aluminosilicate glass consisting essentially of, in weight percent on the oxide basis as calculated from the batch: a from 50 to 75% SiO2; b from 10 to 30% Al2O3; c from 5 to 25 percent total alkali oxide with Na2O comprising at least one-half of the total alkali metal oxide and with from 0 to 6% K2O; d from 0 to 20 percent total alkaline earth metal oxide; e from 0 to 10 percent total of at least one compound selected from the group consisting of La2O3, TiO2, ZrO2, Nb2O5, ZnO, CdO, GeO2, PbO, Bi2O3, CeO2, and B2O3; f. from 0 to 2 percent of at least one compound selected from the group consisting of As2O3 and Sb2O3; g from 0 to 1.5 percent chloride; and h from 0 to 5 percent total of at least one coloring oxide selected from the group consisting of the oxides of Cr, Mn, Fe, Co, Cu, Nd, V, and Ni.
 8. A high strength laminate according to claim 3 wherein the core ply is a spontaneous opal glass consisting essentially of, in weight percent on the oxide basis as calculated from the batch: a from 50 to 75% SiO2; b from 3 to 20% Al2O3; c from 3 to 20% total alkali metal oxide with Na2O comprising from at least one-third of the total alkali metal oxide and with from 0 to 8% K2O; d from 0 to 20 percent total alkaline earth metal oxide; e. from 0 to 10 percent total of at least one compound selected from the group consisting of La2O3, TiO2, ZrO2, Nb2O5, ZnO, CdO, GeO2, PbO, Bi2O3, CeO2, and B2O3; f. from 0 to 2 percent of at least one compound selected from the group consisting of As2O3 and Sb2O3; g. from 0 to 1.5 percent chloride; h. from 0 to 5 percent total of at least one compound selected from the group consisting of the oxides of Cr, Mn, Fe, Co, Cu, Nd, V, and Ni; and i. from 3 to 8 percent fluoride.
 9. A high strength laminate according to claim 3 wherein the core ply is a heat-treatable opal glass consisting essentially of, in weight percent on the oxide basis as calculated from the batch: a. from 50 to 70% SiO2; b. from 15 to 25% Al2O3; c. from 7 to 14% Na2O; d. from 5 to 12% ZnO; e. from 2.5 to 7 percent fluoride; and f. from 0 to 3 percent total of at least one compound selected from the group consisting of As2O3, CaO, MgO, B2O3, Li2O, and BaO.
 10. A high strength laminate according to claim 3 wherein the core ply is a zinc orthosilicate-nucleated nepheline-type glass-ceramic consisting essentially of, in weight percent on the oxide basis as calculated from the batch: a. from 44 to 61% SiO2; b. from 19 to 23% Al2O3; c. from 10 to 14% Na2O; d. from 7 to 10% ZnO; e. from 3 to 6 percent fluoride; and f. from 0 to 3 percent total of at least one compound selected from the group consisting of As2O3, CaO, MgO, B2O3, Li2O, and BaO.
 11. A high strength laminate according to claim 3 wherein the core ply is a titania-nucleated, nepheline-type glass-ceramic consisting essentially of, in weight percent on the oxide basis as calculated from the batch: a. from 50 to 65% SiO2; b. from 20 to 30% Al2O3; c. from 15 to 20 percent total Na2O and CaO with (1) from 9 to 20% Na2O, and (2) from 0 to 9% CaO; d. from 0.25 to 3% MgO; e. from 3 percent to 6 percent TiO2; and f. from 0 to 5 percent total of at least one compound selected from the group consisting of CdO, ZnO, As2O3, K2O, and B2O3.
 12. A high strength glass-ceramic laminate comprising a tensilely-stressed surface layer fused to and substantially enveloping said core portion such that the fused surface between said core portion and said surface layer is essentially defect-free wherein: a. the ratio of the thickness of sAid core portion to the total thickness of said surface layer is at least about 10:1; b. the thickness of said surface layer is at least about 0.002 inch; and c. the coefficient of thermal expansion of said surface layer is at least about 5 X 10 7/* C. less than the coefficient of thermal expansion of said core portion.
 13. A high strength laminate according to claim 12 consisting of a three-ply sheet having a core ply and two adhered plies.
 14. A high strength laminate according to claim 13 wherein the ratio of the thickness of the tensilely stressed ply to the total thickness of the compressively stressed plies adjacent thereto is less than 30:1.
 15. A high strength laminate according to claim 13 wherein: a. the tensilely-stressed core ply is a glass-ceramic selected from the group consisting of beta-spodumene solid solution and rutile, anatase and nepheline solid solution; and b. the compressively-stressed adhered plies are a glass-ceramic selected from the group consisting of beta-quartz solid solution, beta-spodumene solid solution, and rutile, cordierite and beta-eucryptite solid solution. 